Catalysis

ABSTRACT

This invention relates to catalysts and more particularly to the catalyst decomposition of one or more oxides of nitrogen present in a gas stream by contacting the gas with a catalyst comprising a compound of the formula A x  B y , where A is selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th, and U, B is selected from the group consisting of B, C, N and Si, x has a value 1≦x≦7 and y has a value 1≦y≦4.

This is a continuation of application Ser. No. 948,508, filed Oct. 4,1978, which is a continuation of Ser. No. 826,831, filed Aug. 22, 1977,now abandoned.

This invention relates to catalysis. More particularly, it relates tothe catalytic decomposition of one or more oxides of nitrogen,especially when there is also present a stoichiometric excess of oxygenor oxidation species.

According to the invention, a method for the decomposition of one ormore oxides of nitrogen present in a gas stream comprises contacting thegas with a catalyst comprising a compound of formula A_(x) B_(y), whereA is selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr,Mo, W, Th and U, B is selected from the group consisting of B, C, N andSi, x has a value 1≦x≦7 and y has a value 1≦y≦4.

Typical compounds useful as catalysts according to the invention are asfollows:

(i) borides of titanium (TiB₂), zirconium (ZrB₂), molybdenum (MoB) andtungsten (WB);

(ii) carbides of titanium (TiC), silicon (SiC), niobium (NbC),molybdenum (Mo₂ C) and tungsten (W₂ C);

(iii) mitrides of titanium (TiN), tantalum (Ta₂ N), boron (BN) andtungsten (WN₂) and

(iv) silicides of molybdenum (MoSi₂) and tungsten (WSi₂).

The compounds may be used as catalysts either in the supported or theunsupported form. If they are used in the supported form, any of thematerials known as catalyst supports may be used, for example an inertunitary ports refractory ceramic honeycomb. Alternative supports are,for example, pellets, granules, spherules, powder shaped extrudates,monoliths, corrugated metallic substrates and nichome wire.

Suitable materials which may be used to constitute a ceramic support arezircon-mullite, mullite, alpha alumina, sillimanite, magnesiumsilicates, kaolin clays, zircon, petalite, spodumene, cordierite andmost alumoro-silicates.

Proprietary products which may be used are described in U.S. Pat. Nos.3,397,154 and 3,498,927 and British Pat. No. 882,484. Examples of suchproducts are "TORVEX", a mullite honeycomb having eight corrugations perinch and an alumina washcoat; "THERMACOMB", a cordierite honeycombsupplied by the American Lava Corporation and EX 20, a cordieritehoneycomb supplied by Corning Glass.

A surface area of at least 600 sq. ft. per cubic foot of washcoated (ashereinafter defined) honeycomb is preferred. At least 10% of themicropores should have a diameter greater than 10 microns and 90%greater than 10 microns is prefered

Suitable corrugated or extended metallic subtrates which may be used arethose disclosed in our co-pending British Patent Application No.49501/73 dated 24th Oct. 1973 and German DOS No. 2,450,664.

One particular extended metal substrate suitable for use with a catalystof the present invention is an oxidation resistant alloy of ironincluding at least one of the elements (3-40 wt.%) chromium, (1-10 wt.%)aluminium, (a trace to 5 wt.%) cobalt, (a trace to 72 wt.%) nickel and(a trace to 0.5 wt.%) carbon.

Another extended metal substrate is a heat resisting alloy having anaggregate nickel plus chromium content of greater than 20% by weight,the said heat resisting alloy also exhibiting relatively high mechanicalstrength and thermal conductivity.

The substrate may be deposited thereon, as a first coating, a "washcoat"or a layer of a high serface area catalytically active refractory metaloxide which is in turn impregnated or coated with the catalyst materialaccording to the invention.

Preferably, the first refractory metal oxide coating upon the substrateis an adherent oxide coating or film containing, for example, one ormore of alumina, silica, titania, zirconia, hafina, thoria, beryllia,magnesia, chromia, or boria but may also comprise oxygen containinganions such as chromate, phosphate, silicate, and nitrate.

The layer comprising the compound useful as a catalyst according to theinvention is deposited upon the uncoated or washcoated substrate and maybe deposited in a form which possesses catalytic activity or which iscapable of being rendered catalytically active by subsequent treatment.Catalytic structures according to this aspect of the present inventionare extremely robust and effective in catalysing high temperaturereactions such as the decomposition or reduction of nitric acid.

Various compounds suitable for use as catalysts in accordance with theinvention will now be described with reference to the followingexamples:

EXAMPLE 1

Titanium nitride, A sample of this compound, prepared in the laboratoryby heating 0.49 g of titanium in NO at 550° C., was tested for NOremoval in a gas stream containing 1000 ppm NO in N₂ passed through abed depth of 0.5 cm. The flow rate was 100 cm ³ min ⁻¹ which gave anoperating space velocity of 45,000 h ⁻¹. Maximum NO decomposition wasabout 57% at 550° C. under isothermal conditions. Small doses of air(approx. 2,000,000 ppm O₂) did not poison the catalyst. A similarexperiment using 0.28 g of commercially-available titanium nitride(British Titan Products) gave a similar pattern of results; maximum NOdecomposition was about 45% at 570° C. and again there was littledeleterious effect on adding doses of air to the gas stream.

EXAMPLE 2

Tungsten carbide, 0.6 g of tungsten carbide, tested at 800° C. in a gasstream containing 1000 ppm NO in N₂ flowing at 80 ml.min ⁻¹ gave 52%decomposition of NO initially, increasing to 58% after about 41/2 hours.About 20,000 ppm oxygen, added continuously, gave a rapid increase in NOdecomposition, peaking at about 80% and then falling off to stabilize at68%.

Using only 0.4g of similar catalyst at 800° C. in a gas streamcontaining 4000 ppm NO in N₂ at 80 ml.min⁻¹, activity increased slowlyto 84% NO decomposition. Although thane(added continuously at 55 ppm)had no poisoning effect oxygen (20,000 ppm) did reduce the catalyticactivity. However, using a greater weight (2.3g) of similar catalyst atonly 600° C., the addition of oxygen had a beneficial effect on nitricoxide decomposition.

EXAMPLE 3

Molybdenum carbide. 1 g of molybdenum carbide (Murex Ltd) at 550° C. ina gas stream containing 1000 ppm NO in N₂ at a flow-rate of 100 ml.min⁻¹through a bed depth od 1 cm., giving an operating space velocity of22,500h⁻¹, gave about 70% NO decomposition initially, falling off toabout 50% in about 2 hours. Doses of air (2000,000 ppm O₂) had adeleterious effect for the duration of the dose, and continuous additionof 20,000 ppm O₂ had a more serious effect from which the catalyst onlypartially recovered. NO decomposition had fallen to about 25% after twosuch continuous additions, each lasting about 20 minutes. Doses of CO(1,000,000 ppm) and C₂ H₄ (1000 ppm) had no significant effect on NOdecomposition.

EXAMPLE 4

Niobium carbide, 1 g of niobium carbide (Murex Ltd) was tested underconditions similar to those for molybdeneum carbide. Decomposition of NOreached a maximum at 500° C. of about 38% after about an hour, afterwhich the activity fell off. Continuous addition of oxygen had no effecton decomposition.

EXAMPLE 5

Tungsten monoboride. 21 g of tungsten boride was tested in a gas streamcontaining 1,000 ppm NO in N₂ flowing at a rate of 100 ml.min ⁻¹ througha bed depth of 1 cm.; operating space velocity was 20,000h⁻¹. At 150° C.there was a decomposition peak of about 65% and a further peak at 475°C. of about 57%. Under isothermal conditions at 500° C., an initial peakof about 57% was followed by a gradual recession to about 30%. A 2 mldose of air containing about 200,000 ppm O₂ caused a fast recessionfollowed by a recovery to about 38% and a further 4 ml dose of aircaused a recovery preceeded by a quick recession, to about 40%.Continuous addition of 20,000 ppm O₂ caused a rapid increase in activitypeaking at about 85% NO decomposition.

EXAMPLE 6

Tungsten silicide. 2.9 g tungsten silicide, WSi₂ (Koch LightLaboratories) was tested by passing 1000 ppm No in N₂ through a beddepth of 1.4 cm at a rate of 100 ml. min⁻¹. At 560° C. under isothermalconditions, a peak of about 60% NO decomposition was followed by agradual recession to a constant 30% activity. Addition of oxygen had noeffect.

What we claim is:
 1. A method for the decomposition of NO present in agas stream which comprises contacting the gas with a catalyst consistingessentially of a compound selected from the group consisting of TiB₂,ZrB₂, MoB, WB, Ta₂ N, BN, MoSi₂ and WSi₂.
 2. A method according to claim1 wherein the catalyst is supported on a carrier made from a ceramic ormetallic material.
 3. A method according to claim 2 wherein the carrieris in the form of pellets, granules, spherules, powders, wire mesh,monoliths or corrugated metallic substrates.
 4. A method according toclaim 3 wherein the monolith is made from a ceramic material selectedfrom zirconmullite, alpha alumina, sillimanite, magnesium silicates,kaolin clays, zircon, petalite, spodumene, cordierite and mostalumino-silicates.
 5. A method according to claim 3 wherein the monolithis in the form of a honeycomb.
 6. A method according to claim 5including a layer of high surface area catalytically active refractorymetal oxide applied to the carrier prior to application thereto of thecatalyst compound.
 7. A method according to claim 6 wherein the saidrefractory metal oxide is selected from the group consisting of alumina,silica, titania, zirconia, hafnia, thoria, beryllia, magnesia, chromia,or boria but may also comprise oxygen containing anions such aschromate, phosphate, silicate and nitrate.